This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Ewing sarcoma is the second most common form of bone cancer in children and young adults. The t(11;22) chromosomal translocation results in expression of a chimeric fusion protein containing EWS-derived sequences at the amino-terminal region fused to the carboxy-terminal regions of the ETS transcription factor FLI-1. In recent studies, we reported that both the knockdown of endogenous EWS, and expression of the EWS/FLI1 fusion protein in zebrafish embryos and HeLa cells lead to mitotic defects. Mislocalization of a key mitotic regulator, Aurora B kinase, precedes the development of mitotic defects. We subsequently demonstrated a biochemical interaction between EWS/FLI1 and wildtype EWS: this interaction leads to inhibition of EWS activity by EWS/FLI1. The hypothesis of this proposal is that EWS/FLI1 interaction with EWS in Ewing sarcoma induces mitotic defects leading to chromosome instability and malignant transformation. The research focus of this application is to elucidate the molecular mechanism for EWS/FLI1-induced mitotic defects.